A known method of providing a fluid connection is to use a threaded pressure fitting comprising two threaded cooperating parts, wherein tightening the fitting by rotating one part relative to the other results in a seal at a mating interface therebetween. The seal may be formed by compression at a mating interface of the parts of the fitting. One advantage of such fittings is that such connections may be broken by loosening the threaded engagement between the parts, to separate the mating parts of the connection.
A number of different types of threaded pressure fittings exist. One such type of fitting comprises a ferrule which is compressed between a compression nut and pipe when the nut is tightened onto the body of the fitting. Such compression fittings are widely used in compressed air and water connections. A further type of threaded pressure fitting is a flare fitting, in which a flare nut with a female angled surface mates with a corresponding male tapering surface on the body of the fitting. Flare fittings are often used in high pressure systems, or systems requiring a high degree of reliability. Flare fittings may for example be used in aerospace applications and systems carrying potentially hazardous materials such as hydrocarbons (oil, fuel and gas). One industry standard for flare fittings is the Joint Industry Council (JIC) fitting, which has a 37 degree flare mating surface. Other standards for flare pressure fittings include the Aeronautical and Navy (AN) standard and the Society of Automotive Engineers (SAE) standard.
Flare fittings are used extensively in subsea oil and gas development, which generally comprise one or more wells, each having a Christmas tree (often simply referred to as the “tree”). The well tree could more generally be described as a manifold with a plurality of valves to control the flow of the process fluid, which may for example be produced oil or gas from the well, or injected water. The well tree further comprises associated instrumentation to monitor the process fluids. A control system is used to control and monitor the subsea trees, with each tree generally having an associated subsea control module (SCM) mounted on the tree or to a nearby manifold. This control system is controlled from the host facility, typically a fixed or floating oil platform or a floating production storage and offloading (FPSO) facility. The control system is also connected to hydraulic, chemical and electrical feeds from the host facility, which are distributed to each SCM through a network of interconnecting cables and hoses. Typically the hydraulic and chemical hoses are packaged together in bundles with a number of connectors at either hose end being made up simultaneously through the use of a hydraulic stabplate. The interface between the hose connectors and the hoses can be an all-welded construction where the highest levels of reliability is required, but more commonly threaded pressure fittings, such as JIC fittings, are used to allow some reconfiguration or replacement of hoses and connectors in the field without having to can out welding operations which would require the use of specialist equipment and personnel to perform the welding and associated non-destructive testing activities.
A general problem for threaded fittings is the unplanned unfastening or “backing off” of such threaded connections. For example, relative rotation of a connector body and flare nut can result in separation of the flared mating surfaces, resulting in a leak. The connector may unscrew due to vibration of the fitting during transport and/or operation, or as a result of torque applied through hoses (or similar) to one side of the fitting. Helically wound bundles of hydraulic hoses can be particularly problematic, because the application of pressure in the hoses can result in a torque at the fittings.
The consequences of a leaking pressure fitting can be serious. Leaks in vehicle fuel systems may result in fires, and a loss of pressure integrity in a safety critical hydraulic actuation systems (such a vehicle brake system) may endanger life. Leaks in subsea oil and gas development systems can cause pressure in the line to drop significantly and cause a significant loss of fluid to the environment. This loss of pressure or fluid could prevent important hydraulically operated valves from operating completely, leading to consequential environmental damage as well as the loss of safety critical functionality.
It is not possible to use locking washers (or similar) in a compression fitting, since the mating surfaces must engage before any other faces of the fitting and compression/flare nut. In a flare fitting, the flared sealing surface lands out before the adjacent faces of the fitting and flare nut meet.
A known mitigation for this problem is to apply a thread locking compound to the threads before making up the fitting (e.g. Loctite 243). This is not considered good practice in the subsea oil and gas sector due to the potential failure of such materials/compounds in the subsea environment. The compound may also contaminate the sealing surfaces prior to make-up of the fitting, which may create a leak path through the fitting once the compound sets.
Another known method for preventing threaded fittings from backing off is to weld a strip of metal across the nut and body of the fitting after they have been tightened, to mechanically link them. This method is not preferred since it requires a welding operation on a piece of pressure equipment. Such welding operations are difficult to manage and control, are not easily reversed, and the application of heat to the tightened fittings may cause the seal to be broken.
A further known method, used for example in aerospace and motorsport applications, is to use lock wire or safety wire. A hole is provided in a rotatable part of the fitting, (such as a nut) through which the lock wire is threaded, and the lock wire is subsequently wound around a further part (such as the fitting body) so as to prevent backing off of the rotatable parts. This method of securing screw fittings is relatively labour intensive and requires a degree of skill in winding the lock wire to properly constrain rotation. There is also the risk that incorrect installation of the wire with the ‘reverse sense’ could result in it tending slacken the fitting. The operator this needs to know the direction of lock and further to be able to apply this correctly. Whilst this method may suited to the prevention of complete release of a nut from the associated threads, it does not offer sufficient tightness to prevent leakage which could be due to a fractional degree of movement in high pressure systems.
A need exists for a method and device that enables screw fittings to be prevented from unfastening. Preferably such a method should require a minimum of parts, use only standard tools to assemble and disassemble, and be suitable for re-use. Ideally, the parts should be of minimal cost.